1. Field of the Invention
This invention relates to test devices and methods for their use in detecting a characteristic of a sample. In particular, the present invention relates to test devices and methods for their use in performing binding assays such as for determining a ligand in or the liquid binding capacity of a fluid sample, such as a sample of a biological liquid. In preferred embodiment, the present invention relates to test devices for performing binding assays in which a radioisotope is used as a label.
2. Brief Description of the Prior Art
Test devices in the form of test strips have become commonplace in the analysis of various types of samples, such as liquid samples in the nature of industrial fluids, biological fluids, and so forth, because of the convenience and speed of their use. Test strips designed for detecting various clinically significant substances in biological fluids, such as urine and serum, in particular have been found to be very advantageous in assisting the diagnosis and treatment of diseased states in man and other animals.
Conventional test strips generally comprise an absorbent or porous matrix incorporated with indicator reagents, usually of a colorimetric type. The sample to be tested is contacted with the reagent matrix, such as by momentary immersion where the sample is liquid, and the indicator response is observed after a set period of time. Such test strips have the limitation that where more than one chemical reaction is involved, all of the test reactions must be mutually compatible since they all occur in the presence of each other in the reagent matrix.
Recently, certain test strips have been developed which allow several test reactions to take place in a predetermined sequence. U.S. Pat. No. 3,011,874 discloses a test strip representative of this type. The disclosed strip comprises a paper strip partitioned widthwise into various bands, first a blank band for immersion in a liquid test sample, followed successively by a reaction band, a gas liberation band, a barrier band, and an indicator band. However, there still exists a long-felt and recognized need for further improvement.
First of all, the conventional test strips have limited sensitivity. Because the detectable response produced by the conventional strips is almost always a color change, limited color resolution of the eye, and spectrometers as well, does not allow conventional test strips to detect substances, such as hormones, vitamins, and the like, which appear in body fluids at concentrations below 0.1 mg/ml. Secondly, a relatively large sample volume must be provided in order to wet the entire reagent matrix of the conventional test strips. The use of the test strip described in the aforementioned U.S. Pat. No. 3,011,874 requires a sample volume sufficient to wet by capillary absorption all of the strip up to the barrier band.
The known analytical methods for detecting substances which occur in samples in small amounts are based on the binding affinity of such substances for certain synthetic or naturally produced binding agents. The most commonly used binding assay at the present time is probably the radioimmunoassay in which the substance to be determined competes under controlled conditions with a radiolabeled form of itself for binding to a limited quantity of specific antibody. The proportion of the radiolabeled form that successfully binds to antibody to that which remains in a free state is a function of the amount of the substance under determination in the test sample. Because binding assay methods require accurate and timed addition of minute quantities of reagents, the state of the art is that of time consuming and burdensome wet chemistry. The association of binding assay techniques with test devices which attempt to simplify and reduce the cost of such assays has been extremely limited despite the fact that many hundreds of technical papers have appeared in the literature over the past two decades relating to radioimmunoassay approaches alone.
U.S. Pat. No. 3,888,629 describes a device wherein the binding reaction takes place in a disc-shaped matrix pad held in a section of a column. Free and bound label are separated by fitting a wash reservoir column section above the pad containing section and a column section containing an absorbent material below the pad containing section. As the wash solution is drawn through the reaction pad, free label is carried along by the wash leaving the bound label behind because of the filtration properties of the reaction pad. This device would be cumbersome and awkward to use and requires several time consuming manipulative steps.
German Offenlegungsschrift No. 2,241,646 discloses a complex automated instrument for performing radioimmunoassays wherein the binding reagents, i.e. the label and specific antibody, and an aliquot of the liquid to be tested are dispensed onto a cellulose strip at discrete locations. After an incubation period, the test areas on the strip are washed by drawing a liquid therethrough by suction, thereby removing free label. The level of radioactivity remaining at each test area is then measured and related to the amount of unknown in the sample. In addition to requiring the use of an expensive and complex instrument, this method requires controlled dispensing of reagents and timing of incubations, the same as the conventional wet chemistry methods.
It is therefore an object of the present invention to provide a novel test device capable of application to analytical methods wherein a set of sequential test reactions is involved and wherein a minute sample size may be used.
It is a further object of the present invention to provide a test device capable of application to analytical methods having sensitivities below 0.1 mg/ml.
It is a particular object of the present invention to provide a test device useful in performing binding assays to detect characteristics of fluid samples wherein the user is not required to dispense any of the binding reagents or to carefully time incubation periods and particularly wherein radioactive labels are employed, such as in radioimmunoassays.